Method for continuous determination of the properties of a flow of wood fibres for fabrication of fibreboard

ABSTRACT

Methods for the continuous determination of the properties of a flow of wood fibers for use in the production of fiberboard are disclosed including determining reference value characteristics for calibration samples of the fibers, determining predetermined relationships between those reference value characteristics and measured spectral values for the fibers utilizing multivariant statistical regression methods, measuring reflectance spectral values for the fibers illuminated by light, and determining the fiber characteristics relating to the properties from the measured spectral values utilizing the predetermined relationship.

[0001] The present invention relates to a method for continuousdetermination of the properties of a flow of wood fibres for fabricationof fibreboard.

[0002] Properties such as fibre moisture, resin content, opticalcharacteristics and fibre characteristics, such as shives and fibrelength distribution, are generally known to affect the qualityproperties of the finished fibreboard. Appropriate and, in particular,stable fibre moisture facilitates e.g. checks on the pressing phase ofpanel fabrication and creates conditions for good control of the densityprofile of panels.

[0003] After the panel is pressed, the resin added in panel fabricationhardens and creates, with the fibre network, a strong fibre compositestructure. The resin is added either by injection into the blower lineafter the defibering process or into a mechanical resin mixer after thedrying phase. The resin content has a major impact on the finishedpanel's strength properties, such as bending strength and tensilestrength. The resin content is often governed by the specific propertiesthe finished panel is to have. However, an excessive amount of resin isoften dispensed to ensure the desired level of quality, as there is noway to accurately control other factors, such as fibre moisture, fibrelength distribution and density, factors which also affect theproperties of the finished fibreboard.

[0004] To date, the way in which fibre properties (such as the fibrelength distribution and shives) should be specified for the fibrenetwork to be optimal in achieving the desired properties for thefinished fibreboard has been unclear. One reason for this is because ofthe previous unavailability of any simple and, not the least, rapidmeasurement method for determining and characterising fibre lengthdistribution for MDF (medium density fibreboard) fibres. However,equipment is now available for determining and characterising fibreproperties with the aid of image analysis. However, this is a complextechnique, which is only practical in a laboratory environment anddefinitely unsuitable for on-line measurement in the fabrication of MDF.

[0005] Since no facilities have hitherto been available for continuouson-line measurement of parameters for fibre characterisation duringfibreboard fabrication, studying the way in which fibre propertiesshould be devised in order to manufacure ture finished panels with thedesired properties in an optimal fashion has been very difficult. Thisalso applies to efforts to achieve optimal control of the defiberingprocess so the desired fibre properties are attained.

[0006] The need for resin can be minimised and production costs reducedwhen fibre properties are controlled. Resin accounts for about one-thirdof the direct cost of fibreboard fabrication. Resin coating, i.e. thedistribution of resin on the wood fibres, is also affected by thedistribution of fibre lengths. Fine fibre fractions require more resinthan thicker fibres. As a result, increasing resin content does notproduce the anticipated increase in the strength of the finished panelwhen the fibre length distribution of the fibres used is inappropriate.

[0007] Multivariate analysis of spectral data for determining wood chipcomponents, such as Klason lignin, extract and the total amount ofcarbohydrates, is described in “Dialog Information Services,” File 248,PIRA, Dialog accession No. 00393878/5, PIRA accession No. 20017450,Meder R. et al.: “Prediction of wood chip and pulp and paper propertiesvia multivariate analysis of spectral data,” Melbourne, Australia, May2-6 1994, pp. 479-484. Here, Principal Component Analysis (PCA) andPrincipal Component Regression (PCR) were used for analysingnear-infrared spectra (NIR), Fourier-transformed infrared (FTIR) spectraand nuclear magnetic resonance (NMR) spectra taken from examined woodchips.

[0008] WO 97/04299 describes multivariate data analysis of near-infrared(NIR) spectra taken from raw materials, such as sawdust, shavings andwood chips, used for manufacturing chip board. Use of the measuredvalues for controlling panel fabrication is also described.

[0009] The objective of the present invention is to refine thistechnique, based on multivariate analysis of spectral data, so it canalso be used for continuous determination of the properties of woodfibres for fibreboard fabrication, said properties being of decisiveimportance to the properties of the finished panel.

[0010] This objective is achieved with a method according to claim 1.

[0011] Continuous determination of fibre length distribution accordingto the method according to the invention greatly enhances the ability tocontrol the fabrication process and optimise the cost of that process,primarily by always dispensing the exact right amount of resin required.In addition, continuous feedback from the measured fibre lengthdistribution to the defibering process also becomes possible forcontrolling this process. On-line quality control of fibre properties,something hitherto impossible in the fabrication of fibres forfibreboard, is achieved in this way. In addition, determination of thequantity of resin, in the flow of resin-coated fibres, from thereflectance spectrum supplies continuous information on both the totalamount of resin dispensed, a previously per se possible determination,and on the distribution of resin on fibres. The moisture content offibres in the fibre flow, which can be obtained from the reflectancespectrum with the method according to the invention, is an importantparameter in the fabrication of fibreboard. In addition, thebrightness/colour of the fibre flow can be continuously determined. Thebrightness/colour of the fibre flow is also a measure of the effect ofthermal pre-treatment on the chips or fibres and additionally suppliesinformation on the brightness/colour of the finished product.

[0012] According to one advantageous embodiment of the method accordingto the invention, light spectra in the 400-2500 nm wavelength range areused. Primarily, two physical processes make the study of thiswavelength range especially suitable for determining said qualityproperties of the fibre flow, viz. energy absorption and lightscattering. Near-infrared (NIR) spectroscopy is based on electromagneticradiation in the 700 to 2500 nm wavelength range. Mainly organicsubstances containing C—H, O—H and N—H bonds, which absorb theradiation, lie within this range. The energy mainly excites harmonicsand combinations of rotating and vibrating states. Organic materialabsorbs less energy in the NIR range than in the UV and IR range. As aresult, near-infrared light penetrates more deeply into the sample. Anon-homogenous material, such as wood fibres, induces a scattering oflight related to the size of the particles. This property, plusmolecular vibrations and rotations, make it possible to characterise thesize distribution of the fibres in the fibre flow, i.e. to determine thefibre length distribution.

[0013] According to additional advantageous embodiments of the methodaccording to the invention, a reference distribution of fibre length isestablished for calibration samples of the fibre flow. The samecalibration samples are also used for determining fibre lengthdistribution from the measured reflectance spectrum, and these lengthsare modelled in a calibration procedure against said reference fibrelength distribution with the aid of multivariate statistical regressionmethods for determining said predetermined relations between measuredspectral values and fibre length distribution. The reference fibrelength distribution is suitably determined by means of a standardisedmethod. In an analogous way, the predetermined relations betweenmeasured spectral values and reference values for resin content, fibremoisture and optical properties are determined, the reference value forresin content then being appropriately established by determination ofthe nitrogen content of nitrogenous resin by means of the Kjeldahlmethod, the reference value for fibre moisture being determined bydrying and weighing the calibration samples, and the reference value foroptical properties being determined according to the ISO Brightnessnorm.

[0014] According to another advantageous embodiment of the methodaccording to the invention, one or more of the parameters derived fromthe reflectance spectrum, i.e. fibre length distribution, resin content,fibre moisture and optical properties, is/are used in a feedbackprocedure for on-line control of the fibre preparation process in orderto impart the desired properties to the wood fibres used. This makespossible continuous, overriding quality control of the said parameters.

[0015] The invention will now be explained in greater detail, referringto the attached sole FIG. 1, which illustrates an example of themethod's implementation according to the invention.

[0016] The FIGURE depicts mechanic or pneumatic transport 2 of a flow ofwood fibres intended for use in fibreboard fabrication. The wood fibreis suitably prepared in a refiner process with a specific beating energyless than 500 kWh/h. At a measuring point, situated after the defiberingand drying process, the flow of fibres is illuminated with light of anappropriate wavelength, and the ensuing reflectance spectrum is pickedup by a detector 4 in the 400-2500 nm wavelength range, i.e. in thevisible (VIS) and near-infrared (NIR) range. The detected spectrum isA/D converted at 6. A typical conformation for the resulting spectrum isshown at 8 in the FIGURE

[0017] The calibration required in the method according to the inventionis performed at 10 in the FIGURE in the following manner.

[0018] A number of calibration samples of fibre flow are preparedaccording to a test plan in which the level of the test variables isselected to produce a good spread in the values for properties ofinterest, viz. fibre length distribution, resin content, fibre moistureand optical properties. A calibration sample can encompass an operatingperiod in full-scale production.

[0019] Reflectance spectra of the aforementioned kind are recorded forthe calibration samples, and reference values are determined, using anappropriate laboratory method, for each of the aforesaid properties.Thus, fibre length distribution (and shives) can be determined with e.g.image analysis. Resin content can advantageously be determined with theKjeldahl method in instances in which a nitrogenous resin is used. Ife.g. phenol resin, which does not contain nitrogen, is used, some otherappropriate method must be employed for determining reference values forresin. Reference value for fibre moisture can be established using adrying and weighing procedure, and optical properties can be determinedaccording to the ISO Brightness norm for determining light reflection.

[0020] In the calculation unit 12 in the FIGURE, mathematicalcorrelations can be established between spectra recorded for calibrationsamples of fibre flow and the corresponding reference values with theaid of multivariate statistical regression methods, preferably PrincipalComponent Analysis (PCA) and Projection to Latent Structures (PLS)regression. This type of calculation is well-known, cf. e.g. Martens H.and Naes T., “Multivariate Calibration,” Wiley & Sons, New York (1989);Wold S., Johansson E. and Cocchi M., “Partial Least Squares Projectionsto Latent Structures, in QSAR in drug design, “Theory, Methods andApplication,” Kubini H., Ed., (1993); Höskuldsson A., “PLS RegressionMethods,” Journal of Chemometrics, vol. 2 (1988); Geladi P. and KowalskiB. R., Analytica Chimica Acta, 185 (1986) and Wold S., Esbensen K., andGeladi P., Principal Component Analysis, Chemometrics and IntelligentLaboratory Systems, 2 (1987).

[0021] This creates a calibration model or a relation between thereference values and associated spectra, determined with appropriatelaboratory methods, (quality variables (y) as a function of the spectrum(x)y=f(x)), the calibration procedure being carried out on a number offibre flow samples with relatively good variation in the said propertiesor parameter values, as noted above.

[0022] With the aid of relations established in this way, saidparameters can be obtained from subsequently recorded reflectancespectra for the fibre flow. This achieves contactless, on-linemeasurement of fibre length distribution at 20, 22, resin content at 14,fibre moisture at 16 and optical properties at 18 in the FIGURE. Thus,the four cited properties are determined in one and the same measurementprocedure from a spectrum's information content.

[0023] The moisture content of wood fibre is normally 5-20%, often10-20%.

[0024] The length of fibres used in fabricating fibreboard is typically0.154 mm for spruce fibres, and the upper fibre length limit for woodwith shorter fibres, such as beech, is about 3 mm.

[0025] In the method according to the invention, fibre lengthdistribution can be determined at a relatively large number of intervalsfor the lengths range in question. In the 0-7 mm length range, themethod according to the invention can determine fibre lengthdistribution in intervals as small as 0.1 mm, i.e. said length range issubdivided into 70 intervals or fractions, cf. the distribution at 22 inthe FIGURE.

[0026] The three parameters fibre length distribution, resin content andfibre moisture are of major importance for controlling quality andoptimising the cost of fibreboard fabrication, as discussed above.Optical properties are also of interest, as the lightest possible boardis normally desirable. Thus, this continuous, on-line measurementaccording to the method according to the invention makes possiblefeedback and on-line quality control of fibre properties, somethinghitherto impossible in the fabrication of fibreboard.

1. A method for continuous determination of the properties of a woodfibre flow for fibreboard fabrication, characterized in that a fibreflow sample is illuminated with light, and, from measured ensuringreflectance spectrum, one or more of the properties fibre lengthdistribution, resin content, fibre moisture and optical properties forthe fibre flow sample is/are determined from predetermined relationsbetween measured spectral values and the absolute magnitudes of saidproperties.
 2. The method according to claim 1, characterized in thatlight spectra in the 400-2500 nm wavelength range are used.
 3. Themethod according to claim 1 or 2, characterized in that a referencefibre length distribution is determined for calibration samples of thefibre flow, and a fibre length distribution, determined from thereflectance spectrum measured for the same calibration samples, ismodelled in a calibration procedure against said reference fibre lengthdistribution with the aid of multivariate statistical regression methodsfor determining said predetermined relations between measured spectralvalues and fibre length distribution.
 4. The method according to any ofthe previous claims, characterized in that a reference fibre moisture isdetermined for calibration samples of the fibre flow, and referencefibre moisture, determined from the reflectance spectrum measured forthe same calibration samples, is correlated in a calibration procedureagainst said reference fibre moisture with the aid of multivariatestatistical regression methods for determining said predeterminedrelations between measured spectral values and fibre moisture.
 5. Themethod according to any of the previous claims, characterized, in that areference resin content is determined for calibration samples of thefibre flow, and spectral values, determined from the reflectancespectrum measured for the same calibration samples, are correlated in acalibration procedure against said reference resin content with the aidof multivariate statistical regression methods for determining saidpredetermined relations between measured spectral values and resincontent.
 6. The method according to any of the previous claims,characterized in that reference values for optical characteristics aredetermined for calibration samples of the fibre flow, and spectralvalues, determined from the reflectance spectrum measured for the samecalibration sample, are correlated in a calibration procedure againstsaid optical reference values with the aid of multivariate statisticalregression methods for determining said relations between spectralvalues and optical properties.
 7. The method according to any of claims3-6, characterized in that calibration samples are prepared according toa test plan with varying levels for test variables selected to produce agood spread of fibre length distribution, fibre moisture, resin contentand optical properties in the different calibration samples.
 8. Themethod according to any of the previous claims, characterized in thatone or more of the parameters fibre length distribution, resin content,fibre moisture and optical properties, determined from the reflectancespectrum, is/are used in a feedback procedure for on-line control of thefibre preparation process in order to impart the desired properties tothe wood fibre used.
 9. The method according to claim 8, whereby thefibre is produced in a refining process, characterized in that one ormore of the parameters fibre length distribution, resin content, fibremoisture and optical properties is/are fed back to control of thedefibering process.